FIG. 1 shows a cross-sectional structure of a conventional semiconductor waveguide for use in a photo detector.
The semiconductor waveguide forms a semiconductor lamination structure by sequentially forming a n type cladding layer, a low carrier concentration light absorption layer, a p type cladding layer and a contact layer on a semiconductor substrate in due order.
Those layers provided above the n type cladding layer are of mesa stripe shape extending in the optical waveguide direction (which is the direction perpendicular to the drawing sheet). Side surfaces of the mesa stripe structure are each covered with a dielectric film. An electrode (not shown) is mounted on the contact layer and an electrode (not shown) on the back surface of the semiconductor substrate, thereby constituting a photo detector as a whole.
The band gap energy of the light absorption layer is set lower than that of the p type cladding layer situated above and that of the n type cladding layer situated below, and a signal light incident on the light incidence end face of the light absorbing layer is guided to the light absorption layer.
If reverse voltage is applied between the p type cladding layer and the n type cladding layer, then a depletion layer is formed within the low carrier concentration light absorption layer and the signal light guided to the light absorption layer is converted into a photoelectric signal by the function of the high electric field generated within the aforementioned depletion layer.
That is, in the conventional photo detector, excitation carriers within the depletion layer generated by the incident signal light are detected as photoelectric current. The excitation carriers are separated and drifted by the electric field generated within the depletion layer. In case of holes, the carriers reach the p type cladding layer. In case of electrons, the carriers reach the n type cladding layer. In both cases, the excitation carriers contribute to photoelectric current.
The reason the lamination structure of the semiconductor device is made to be of mesa stripe shape is to increase the operating speed of the photo detector.
Specifically, to increase the operating speed of the photo detector, it is necessary to decrease electric capacity generated within the depletion layer at the time of applying reverse voltage as mentioned before. This can be effectively realized by reducing the cross-sectional area of the light absorption layer and therefore that of the depletion layer formed during the application of reverse voltage.
To realize this, in principle, only the light absorption layer might be etched to have a mesa stripe shape. In practice, however, as shown in FIG. 1, all portions provided above the n type cladding layer are etched to be mesa stripe shaped. Sometimes, part of the n type cladding layer is simultaneously etched as well as those portions.
In addition, the dielectric film is provided for decreasing leak current flowing through the side surface of the mesa stripe structure during the operation of the photo detector.
It is known that leak current flows by way of the surface level and defects of the side surface of the mesa stripe structure. It is thus possible to prevent the surface level and defects from causing the leak current to flow by coating the mesa stripe side surface with a dielectric film.
Meanwhile, in the photo detector as mentioned above, for purposes of making the light enter into the device effectively, light incidence end faces are usually provided by cleavage. The light incidence end face formed by cleavage is flat in terms of atom level and light can enter the waveguide without scattering. The light incidence end face is coated with a nonreflective film made of SiO.sub.2 or SiNx to prevent the reflection of the incident light.
The biggest problem with the photo detector of the above-described structure is that leak current flows through side surfaces of the mesa stripe structure while the photo detector is in operation.
As described above, to prevent the leak current, side surfaces thereof are coated with the dielectric films. However, such coating does not always prevent leak current sufficiently.
Furthermore, to the best of the present inventors' knowledge, considerable high leak current flows even on the cleaved light incidence end face.
It is therefore an object of the present invention to provide a novel semiconductor waveguide type photo detector capable of preventing leak current from flowing on mesa stripe side surfaces and cleaved light incidence end faces while the device is in operation.
It is another object of the present invention to provide a method of manufacturing the aforesaid semiconductor waveguide type photo detector.